I would go with Biloxi on this one...more of a 'give us a few days notice when you will be available/have a sitter/etc'...because even though I have children, they are not with us all the time...but that doesn't mean that we can/will meet up with people at the drop of a hat.
Courtesy goes both ways.
Of course, kids do make convienent excuses as well...and they are also prime cootie carriers (ah, germs from school...the gift that keeps on giving
)...
I wouldn't think it too pushy if someone asked to nail down a slightly more general time frame...but that's just me. YMMV.